1. Field of the Invention
This invention relates to plant disease resistance. More specifically, the invention relates to transgenic plants having increased resistance to plant disease.
2. Description of the Related Art
Lignin is the major structural component of secondarily thickened plant cell walls. It is a complex polymer of hydroxylated and methoxylated phenylpropane units, linked via oxidative coupling that is probably catalyzed by both peroxidases and laccases (Boudet et al., 1995). Lignin imparts mechanical strength to stems and trunks, and hydrophobicity to water-conducting vascular elements.
There is considerable interest in the potential for genetic manipulation of lignin levels and/or composition to help improve digestibility of forages and pulping properties of trees. Key enzymes in lignin biosynthesis, including caffeic acid 3-O-methyltransferase (also known as caffeic acid/5-hydroxyferulic acid O-methyltransferase) (COMT) and caffeoyl CoA 3-O-methyltransferase (CCoAMT), have been targets of these efforts (Meyermans et al., 2000; Zhong et al., 2000; Guo et al., 2001a; Ye et al., 1994, 2001; Do et al., 2007). COMT and CCoAMT suppressed alfalfa lines have increased digestibility (Guo et al., 2001a,b). See also PCT Publication WO 01/73090, which discloses methods of modifying plant lignin content and composition by transformation with DNA constructs that affect expression of COMT or CCoAMT.
Phymatotrichopsis Root Rot (PRR), also called Cotton Root Rot, is one of most destructive diseases of cotton (Gossypium spp.), alfalfa (Medicago sp.), and many other dicotyledonous plants. PRR is caused by a soil-borne fungus, Phymatotrichopsis omnivora (Duggar) Hennebert (1973), (formerly termed Phymatotrichum omnivorum) and causes significant economic losses every year in the United States. Hennebert (1973) named the fungus as P. omnivora (Duggar) Hennebert to emphasize its morphological affinity to Botrytis-like Ascomycetes. P. omnivora has a very broad host range and attacks almost 2,000 dicotyledonous species, but interestingly it does not cause disease on monocotyledonous plant species, including maize and sorghum. The disease is economically important in cotton, alfalfa, beans, peanut, sweet potatoes, ornamental shrubs, and fruit, nut, and shade trees (Lyda, 1978; Lyda and Kenerly, 1993; Streets and Bloss, 1973). Other important fungal diseases of, for instance, Medicago, include Anthracnose (caused by Colletotrichum trifolii), Phoma Black Stem (caused by Phoma medicaginis), Phytophthora Root Rot (caused by Phytophthora medicaginis, also known as Phytophthora megasperma f sp. medicaginis or Phytophthora sojae f sp. medicaginis), Sclerotinia Crown and Stem Rot (caused by Sclerotinia sclerotiorum), and Aphanomyces Root Rot (caused by Aphanomyces euteiches Drechs.) (Graham et al., 1979; Tivoli et al., 2006), among others.